Wow, Beagle, it's amazing how tastes differ - I'm a big fan of this recording, and of the Carmina's recorded repertoire in general. They have a lot of "air" and "fire" energy (think Martha Argerich), which, in the case of their Opus 3 could conceivably be so light-footed that it doesn't register on someone's pleasure-o-meter - for instance, my great aunt Dorothy, who loves to read Opus 3 at half tempo with her amateur quartet. Otherwise, I simply can't understand how you fail to respond to this elegant, charming rendition - in fact, the Opus 3 is for me the highlight of a very lively, sunny, unusual recording.
In terms of the recorded sound, this is indeed somewhat "sharper" than some of their best recordings (sounds like a small recording space?); even so, the signature sweetness of the Carminas I know from their decades recording for DENON shines through.
I don't think the Carminas use baroque instruments, but they are certainly at the forefront of something like the "post Harnoncourt" wave of musicians: they have swallowed the baroque movement whole, digested it, and are currently producing their own inimitable creative product, which seems not to be to quite everyone's taste (for example, again, my great aunt Dorothy, who also loves her vibrato), but is dynamic and unforgettable. The experimental nature of this recording (including the unconventional integration of the guitar) is a departure from the Carmina's traditional DENON recordings, but a welcome change on the otherwise rather stuffy quartet scene.